Return string of text matched by last search, without text properties.
NUM specifies which parenthesized expression in the last regexp.
Value is nil if NUMth pair didn't match, or there were less than NUM pairs.
Zero means the entire text matched by the whole regexp or whole string.
STRING should be given if the last search was by `string-match' on STRING.
If STRING is nil, the current buffer should be the same buffer
the search/match was performed in.